The present invention relates generally to steerable catheters, and more specifically to steerable electrophysiology catheters for use in mapping and ablation of the heart.
The heart includes a number of pathways which are responsible for the propagation of signals necessary for normal, electrical and mechanical function. The present invention is concerned with treatment of tachycardia, abnormally rapid rhythms of the heart caused by the presence of an arrhythmogenic site or accessory pathway which bypasses or short circuits the normal pathways in the heart. Tachycardias may be defined as ventricular tachycardias (VTs) and supraventricular tachycardias (SVTs). VTs originate in the left or right ventricle and are typically caused by arhythmogenic sites associated with a prior myocardial infarction. SVTs originate in the atria and are typically caused by an accessory pathway.
Treatment of both ventricular and supraventricular tachycardias may be accomplished by a variety of approaches, including drugs, surgery, implantable pacemakers/defibrillators, and catheter ablation. While drugs may be the treatment of choice for many patients, drugs typically only mask the symptoms and do not cure the underlying cause. Implantable devices, on the other hand, usually can correct an arrhythmia only after it occurs. Surgical and catheter-based treatments, in contrast, will actually cure the problem usually by ablating the abnormal arrhythmogenic tissue or accessory pathway responsible for the tachycardia. The catheter-based treatments rely on the application of various destructive energy sources to the target tissue including direct current electrical energy, radiofrequency electrical energy, laser energy, and the like.
Of particular interest to the present invention, are radiofrequency (RF) ablation protocols which have proven to be highly effective in tachycardia treatment while exposing the patient to minimum side effects and risks. Radiofrequency catheter ablation is generally performed after an initial mapping procedure where the locations of the arrhythmogenic sites and accessory pathways are determined. After mapping, a catheter having a suitable electrode is introduced to the appropriate heart chamber and manipulated so that the electrode lies proximate the target tissue. Radiofrequency energy is then applied through the electrode to the cardiac tissue to ablate a region of the tissue which forms part of the arrhythmogenic site or the accessory pathway. By successfully destroying that tissue, the abnormal signaling patterns responsible for the tachycardia cannot be sustained. Methods and systems for performing RF ablation by controlling temperature at the ablation site are described in co-pending application Ser. No. 07/866,683 entitled "Method and System for Radiofrequency Ablation of Cardiac Tissue," filed Apr. 10, 1992, the complete disclosure of which is hereby incorporated by reference.
Catheters designed for mapping and ablation frequently include a number of individual electrode bands mounted to the distal tip of the catheter so as to facilitate mapping of a wider area in less time, or to improve access to target sites for ablation. Such catheters are described in co-pending application Ser. No. 07/866,383, filed Apr. 10, 1992, the complete disclosure of which is incorporated herein by reference. As described in that application, it is frequently desirable to deflect tile distal tip of the catheter into a non-linear configuration such as a semicircle, which facilitates access to substantially all of the heart walls to be mapped or ablated. Such deflection may be accomplished through the use of pull wires secured to the distal tip which can be tensioned from the proximal end of the catheter to deflect the tip in the desired configuration. In addition, mapping and ablation catheters may facilitate rotational positioning of the distal tip, either by rotating the entire catheter from the proximal end, or, in the catheter described in co-pending application Ser. No. 07/866,383, by exerting torque on a core wire secured to the distal tip without rotating the catheter body itself.
Catheters utilized in radiofrequency ablation are inserted into a major vein or artery, usually in the neck or groin area, and guided into the chambers of the heart by appropriate manipulation through the vein or artery. Such catheters must facilitate manipulation of the distal tip so that the distal electrode can be positioned against the tissue region to be ablated. The catheter must have a great deal of flexibility to follow the pathway of the major blood vessels into the heart, and the catheter must permit user manipulation of the tip even when the catheter is in a curved and twisted configuration. Because of the high degree of precision required for proper positioning of the tip electrode, the catheter must allow manipulation with a high degree of sensitivity and controllability. In addition, the distal portion of the catheter must be sufficiently resilient in order to be positioned against the wall of the heart and maintained in a position during ablation without being displaced by the movement of the beating heart. Along with steerability, flexibility, and resiliency, the catheter must have a sufficient degree of torsional stiffness to permit user manipulation from the proximal end.
While mapping and ablation catheters having the forementioned deflectability and steerability have had promising results, such catheters suffer from certain disadvantages. One such disadvantage is the inability to select a desired curvature of deflection in the distal tip. In known catheters, the curvature in the distal tip is determined by the degree of bending stiffness of the distal tip and the degree of tension exerted on the pull wires coupled to it. In any one catheter, the curvature achieved in the distal tip will be the same for any given amount of tension exerted on the pull wires. Thus, if the user desires a particular shape in the distal tip, for example, a semicircle, a particular amount of tension must be exerted on the pull wires, and the semicircular curvature assumed by the distal tip will always have the same radius. Because of the variation in the size of the heart among various patients, as well as the various locations in which a mapping or ablation site may be disposed, it may be discovered during a procedure that the curvature of a given catheter is unsuitable, requiring the catheter to be removed from the patient and replaced with another catheter of suitable configuration.
A further disadvantage of known mapping and ablation catheters relates to the rotatability imparted by the core wire coupled to the distal tip. When the distal tip is deflected in a non-linear configuration, rotation of the core wire will rotate the distal tip about a longitudinal axis parallel to the catheter shaft. However, the rotation or twisting of the core wire relative to the distal tip tends to cause the distal tip to rotate in an irregular motion, wherein the distal end of the deflectable tip catheter may move significantly in longitudinal (axial) position depending upon its rotational position and also out of plane with the axis of the catheter shaft. Such irregular and variable motion defines a non-predictable path and complicates the task of accurately positioning the distal tip near a target site. The movement of the tip along such a non-predictable path hinders mapping of a circular structure, such as mitral or tricuspid valve annulus.
For these and other reasons, a steerable electrophysiology catheter for use in mapping and ablation is desired which facilitates selective adjustment of the curvature of the distal tip, and which has improved positionability, particularly in rotational positioning. More specifically, the electrophysiology catheter should permit adjustment of the curvature of the deflectable tip without removing the catheter from the patient. In addition, when the distal tip is in a deflected configuration, the catheter should be rotationally positionable without rotating its proximal end. This would permit fine control of tip positions without gross rotational movements of the shaft. Further, the end of the tip should rotate in a circle substantially within a single plane perpendicular to the catheter shaft. The catheter should further have the steerability, flexibility, resilience and torsional stiffness required for transluminal positioning in the heart and accurate guidance of the electrodes to a target site.